Furthermore, adversaries get to directly test against the filter. It's like writing a computer virus. Anti-virus software does not help against even rewritten v

Furthermore, adversaries get to directly test against the filter. It's like writing a computer virus. Anti-virus software does not help against even rewritten viruses because every virus writer tests every virus against every antivirus they get their hands on, and they retry, changing something, until it passes. Therefore, no antivirus ever catches a new virus. Same problem with copyright filters. Plenty of bad actors spend days, weeks or months finding (even automated) ways to bypass them.